JP6475837B2 - High strength steel material excellent in brittle crack propagation resistance and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel material excellent in brittle crack propagation resistance and a method for producing the same.

In recent years, in designing structures used in the domestic, foreign, marine, architectural, and civil engineering fields, development of extra heavy steel having high strength properties has been demanded.
When using high-strength steel when designing structures, the structure can be reduced in weight, not only providing economic benefits, but also reducing the thickness of the steel sheet, so that processing and welding Ease of work can be ensured at the same time.
Generally, in high-strength steel, the total rolling reduction decreases during the production of extra-thick materials, and sufficient deformation is not possible as compared with thin materials. As a result, the microstructure of extra-heavy materials becomes coarse. The low-temperature physical properties that the crystal grain size has the greatest influence are lowered.
In particular, in the case of brittle crack propagation resistance, which indicates the stability of structures, there is an increasing number of cases that require assurance when applied to main structures such as ships, but when the microstructure becomes coarse, brittle crack propagation resistance As a result, a phenomenon in which the strength of the steel is extremely lowered is extremely difficult to improve the brittle crack propagation resistance phase of the high-thickness high-strength steel material.

On the other hand, in the case of a high strength steel having a yield strength of 390 MPa or more, in order to improve the resistance to brittle crack propagation, surface cooling is applied during finish rolling for grain refinement of the surface layer, and bending stress is applied during rolling. Various technologies, such as particle size adjustment by, were introduced.
However, in the case of the above-described technique, although it is advantageous for refining the structure of the surface layer part, since the reduction in impact toughness due to the remaining coarse structure excluding the surface layer part cannot be solved, the fundamental to brittle crack propagation resistance It is hard to say that it is a countermeasure.
Moreover, it can be said that the technology itself is unreasonable for commercial application because it is expected that the productivity itself will greatly decrease when the technology itself is applied to a general mass production system.

According to one aspect of the present invention, there is an object to provide a high-strength steel material excellent in brittle crack propagation resistance.
Another object of the present invention is to provide a method for producing a high-strength steel material having excellent brittle crack propagation resistance.

  The high-strength steel material excellent in brittle crack propagation resistance of the present invention is C: 0.05-0.1%, Mn: 1.5-2.2%, Ni: 0.3-1. 2%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.5%, Si: 0.1-0.3%, P: 100 ppm Hereinafter, S: 40 ppm or less, balance Fe, and other inevitable impurities, ferrite single phase structure, bainite single phase structure, ferrite and bainite composite structure, ferrite and pearlite composite structure, and ferrite, bainite and pearlite It has a fine structure including one structure selected from the group consisting of composite structures, and has a thickness of 50 mm or more.

The contents of Cu and Ni are set so that the Cu / Ni weight ratio is 0.6 or less, preferably 0.5 or less.
The steel material preferably has a grain size of 15 μm (micrometer size) having a high tilt boundary with a difference in crystal orientation measured by the EBSD method from the surface layer portion to the steel material thickness ¼ part in the thickness direction of the steel material of 15 degrees or more. M) or less.

The steel material has an area ratio of (100) plane of 30% or more that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction from the surface layer portion to ¼ part of the steel thickness in the thickness direction of the steel material. Preferably there is.
The steel material preferably has a yield strength of 390 MPa or more, and the Charpy fracture surface transition temperature at the surface layer portion and the steel material thickness 1/4 t in the thickness direction of the steel material is preferably −40 ° C. or lower.

The method for producing a high-strength steel material excellent in brittle crack propagation resistance according to the present invention is, by weight, C: 0.05 to 0.1%, Mn: 1.5 to 2.2%, Ni: 0.3. -1.2%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.5%, Si: 0.1-0.3%, P: 100 ppm or less, S: 40 ppm or less, slab containing the remainder Fe and other inevitable impurities was reheated to 950-1100 ° C., and then roughly rolled at a temperature of 1100-900 ° C. comprising the steps of obtaining a bar (bar) to _{Ar 3 + 30 ℃ ~Ar 3 -30} ℃ temperature finish rolling with a thickness of 50mm or more of the steel sheet in the range, the steps of cooling the steel sheet to a temperature of 700 ° C. or less, the It is characterized by that.

For the final three passes during rough rolling, the rolling reduction per pass is preferably 5% or more, and the total cumulative rolling reduction is preferably 40% or more.
After rough rolling, the size of crystal grains in the 1/4 t portion (here, t: steel plate thickness) of the bar before finish rolling can be 150 μm or less, preferably 100 μm or less, more preferably 80 μm or less.

The reduction ratio during finish rolling may be set so that the ratio of slab thickness (mm) / steel plate thickness after finish rolling (mm) is 3.5 or more, preferably 3.8 or more.
It is preferable to cool the steel sheet at a cooling rate of the central portion of 1.5 ° C./s or more.
The steel sheet may be cooled at an average cooling rate of 2 to 300 ° C./s.

Furthermore, the means for solving the above-described problems are not all of the features of the present invention.
The various features of the present invention and the advantages and effects thereof can be understood in more detail through the following specific embodiments.

  According to the present invention, a high-strength steel material excellent in high yield strength and excellent brittle crack propagation resistance can be obtained.

It is the photograph which observed the thickness center part of the invention steel 6 with the optical microscope.

The inventors of the present invention have conducted research and experiments in order to improve the yield strength and brittle crack propagation resistance of thick steel materials having a thickness of 50 mm or more, and have proposed the present invention based on the results. .
The present invention further improves the yield strength and brittle crack propagation resistance of a thick steel material by controlling the steel composition, structure, texture, and manufacturing conditions of the steel material.

The main concept of the present invention is as follows.
1) The steel composition is appropriately controlled in order to improve the strength by solid solution strengthening. In particular, the contents of Mn, Ni, Cu, and Si are optimized for solid solution strengthening.
2) The steel composition is appropriately controlled in order to improve the strength by improving the hardenability. In particular, in order to improve the curing ability, the contents of Mn, Ni, and Cu are optimized together with the carbon content.
By improving the hardenability in this way, a fine structure is secured up to the center of a thick steel material of 50 mm or more even at a slow cooling rate.

3) Preferably, in order to improve strength and brittle crack propagation resistance, the structure of the steel material can be refined. In particular, the structure in the region from the surface layer portion to the steel material thickness ¼ part is refined in the thickness direction of the steel material.
Thus, by refine | miniaturizing the structure | tissue of steel materials, the production | generation and propagation of a crack are suppressed to the minimum with the improvement of the intensity | strength by crystal grain reinforcement | strengthening, and a brittle crack propagation resistance improves.
4) Preferably, the texture of the steel material can be controlled in order to improve the brittle crack propagation resistance.
The crack is parallel to the rolling direction in consideration of the propagation in the width direction of the steel material, that is, the direction perpendicular to the rolling direction, and the brittle fracture surface of the body-centered cubic structure (BCC) being the (100) plane. The area ratio of the (100) plane that forms an angle within 15 degrees with respect to the plane is maximized.

In particular, the texture in the region from the surface layer part to 1/4 part of the steel material thickness is controlled in the thickness direction of the steel material.
The (100) plane that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction serves to block the propagation of cracks.
In this way, by controlling the texture of the steel material, even if a crack occurs, the propagation of the crack is interrupted, and the brittle crack propagation resistance is improved.

5) Preferably, rough rolling conditions can be controlled in order to further refine the structure of the steel material.
In particular, a fine structure is ensured by controlling the rolling conditions during rough rolling.
6) The finish rolling conditions are controlled in order to further refine the structure of the steel material. In particular, by controlling the finish rolling temperature and reduction conditions, very fine ferrite is generated in the grain boundaries and inside the crystal grains by deformation-induced transformation during finish rolling, and the fine center of the steel material is fine. Organization is secured.

Hereinafter, the high-strength steel material excellent in brittle crack propagation resistance which is one aspect of the present invention will be described in detail.
The high-strength steel material excellent in brittle crack propagation resistance which is one aspect of the present invention is C: 0.05 to 0.1%, Mn: 1.5 to 2.2%, Ni: 0.00%. 3 to 1.2%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Cu: 0.1 to 0.5%, Si: 0.1 to 0.3% , P: 100 ppm or less, S: 40 ppm or less, remaining Fe, and other inevitable impurities, and ferrite single phase structure, bainite single phase structure, ferrite and bainite composite structure, ferrite and pearlite composite structure, and ferrite And a fine structure including one structure selected from the group consisting of a composite structure of bainite and pearlite.

Hereinafter, the steel components and component ranges of the present invention will be described.
C (carbon): 0.05 to 0.10% (Hereinafter, the content of each component means% by weight.)
Since C is the most important element for ensuring basic strength, it is necessary to be contained in the steel within an appropriate range. It is preferable to add 05% or more.
However, if the C content exceeds 0.10%, the low temperature toughness is reduced due to the formation of a large amount of island martensite, the high strength of the ferrite itself, and the large amount of low temperature transformation phase. It is preferable to limit to 0.05 to 0.10%, more preferably 0.059 to 0.081%, and still more preferably 0.065 to 0.075%.

Mn (manganese): 1.5-2.2%
Mn is a useful element that improves strength by solid solution strengthening and improves curability so that a low-temperature transformation phase is generated. Further, since the low temperature transformation phase can be generated even at a slow cooling rate by improving the curing ability, it is a main element for securing the strength of the central part of the extra-thick material.
Therefore, in order to acquire such an effect, it is preferable to add 1.5% or more.
However, if the content of Mn exceeds 2.2%, the increase of excessive hardening ability promotes the formation of upper bainite and martensite, and lowers impact toughness and brittle crack propagation resistance.
Therefore, the Mn content is preferably limited to 1.5 to 2.2%, more preferably 1.58 to 2.11%, and even more preferably 1.7 to 2.0%.

Ni (nickel): 0.3-1.2%
Ni is an important element for improving the impact toughness by facilitating cross slip of dislocations at low temperatures and improving the strength by improving the hardenability. To obtain such effects Is preferably added in an amount of 0.3% or more. However, when Ni is added in an amount of 1.2% or more, the curability is excessively increased, a low temperature transformation phase is generated, and the toughness is reduced, and Ni is more expensive than other curability elements, Since the manufacturing cost can be increased, the upper limit of the Ni content is preferably limited to 1.2%.
A more preferable range of the Ni content is 0.45 to 1.02%, and further preferably 0.55 to 0.95%.

Nb (Niobium): 0.005-0.1%
Nb precipitates in the form of NbC or NbCN and improves the base material strength.
In addition, Nb solid-dissolved at a high temperature during reheating precipitates very finely as an NbC form during rolling, and has the effect of refining the structure by suppressing recrystallization of austenite.
Therefore, Nb is preferably added in an amount of 0.005% or more. However, if excessively added, there is a possibility of causing brittle cracks in the corners of the steel material, so the upper limit of Nb content should be limited to 0.1%. Is preferred.
A more preferable range of the Nb content is 0.012 to 0.031%, and further preferably 0.017 to 0.025%.

Ti (titanium): 0.005 to 0.1%
Ti is a component that precipitates as TiN during reheating and greatly improves low-temperature toughness by suppressing the growth of crystal grains in the base material and the weld heat affected zone, and in order to obtain such an addition effect, It is preferable to add 0.005% or more.
However, if Ti is added in excess of 0.1%, the low temperature toughness may decrease due to clogging of the continuous casting nozzle or crystallization at the center, so the Ti content is 0.005 to 0.1%. It is preferable to limit to.
A more preferable range of Ti content is 0.011 to 0.023%, and further preferably 0.014 to 0.018%.

P: 100 ppm or less, S: 40 ppm or less P and S are elements for inducing brittleness by inducing brittleness or forming coarse inclusions at grain boundaries, and for improving brittle crack propagation resistance. P is preferably limited to 100 ppm or less and S: 40 ppm or less.

Si: 0.1 to 0.3%
Si is a substitutional element that improves the strength of steel by solid solution strengthening, has a strong deoxidation effect, and is an essential element for the production of clean steel. Is preferred. However, if added in a large amount, a coarse island-like martensite (MA) phase can be generated and brittle crack propagation resistance can be lowered, so the upper limit of the Si content is preferably limited to 0.3%.
A more preferable range of the Si content is 0.16 to 0.27%, and further preferably 0.19 to 0.25%.

Cu: 0.1 to 0.5%
Cu is a major element that improves the hardenability and causes solid solution strengthening to improve the strength of steel materials. When applied to tempering, Cu is a major element that increases yield strength by generating epsilon Cu precipitates. Since it is an element, it is preferable to add 0.1% or more. However, if added in a large amount, slab cracks due to hot shortness may occur in the steel making process, so the upper limit of the Cu content is preferably limited to 0.5%.
A more preferable range of the Cu content is 0.19 to 0.42%, and further preferably 0.25 to 0.35%.
The Cu and Ni contents may be set such that the Cu / Ni weight ratio is 0.6 or less, preferably 0.5 or less.
If the Cu / Ni weight ratio is set as described above, the surface quality can be further improved.

The remaining component of the present invention is iron (Fe).
However, in a normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and this cannot be excluded.
Since these impurities are well known to those skilled in the art, the entire contents are not specifically mentioned herein.

The steel material of the present invention has a single structure selected from the group consisting of a ferrite single phase structure, a bainite single phase structure, a composite structure of ferrite and bainite, a composite structure of ferrite and pearlite, and a composite structure of ferrite, bainite and pearlite. It has a fine structure.
The ferrite is preferably polygonal ferrite or acicular ferrite, and the bainite is preferably granular bainite.
For example, as the content of Mn and Ni increases, the fraction of acicular ferrite and granular bainite increases, and the strength increases accordingly.

When the microstructure of the steel material is a composite structure containing pearlite, the pearlite fraction is preferably limited to 20% or less.
The steel material preferably has a grain size of 15 μm (micrometer size) having a high tilt boundary with a difference in crystal orientation measured by the EBSD method from the surface layer portion to the steel material thickness ¼ part in the thickness direction of the steel material of 15 degrees or more. Meter) or less.
Thus, in the thickness direction of the steel material, the grain size of the crystal grains having a high tilt boundary where the difference in crystal orientation measured by the EBSD method from the surface layer part to 1/4 part of the steel material is 15 degrees or more is 15 μm (micrometer). ) By miniaturizing as follows, strength is improved by strengthening the crystal grains, crack generation and propagation are minimized, and brittle crack propagation resistance is improved.

The steel material preferably has an area ratio of (100) plane of 30% or more that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction from the surface layer portion to ¼ part of the plate thickness in the thickness direction of the steel material. It may be.
The main reason for controlling the texture as described above is as follows.
The crack is propagated in the width direction of the steel material, that is, the direction perpendicular to the rolling direction, and the brittle fracture surface of the body-centered cubic structure (BCC) is the (100) plane.
Therefore, the present invention is such that the area ratio of the (100) plane that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction is maximized.
In particular, the texture in the region from the surface layer part to 1/4 part of the steel material thickness is controlled in the thickness direction of the steel material.
The (100) plane that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction serves to block the propagation of cracks.
In this way, the texture of the steel material, in particular, the (100) surface forming an angle of 15 degrees or less with respect to the surface parallel to the rolling direction from the surface layer part to ¼ part of the plate thickness in the thickness direction of the steel material. By controlling the area ratio to 30% or more, even if a crack occurs, the propagation of the crack is blocked, and the brittle crack propagation resistance is improved.
The steel material preferably has a yield strength of 390 MPa or more.
The steel material has a thickness of 50 mm or more, preferably a thickness of 50 to 100 mm, more preferably a thickness of 80 to 100 mm.

Hereinafter, a method for producing a high-strength steel material excellent in brittle crack propagation resistance, which is another aspect of the present invention, will be described in detail.
The manufacturing method of the high strength steel material excellent in the brittle crack propagation resistance which is another aspect of the present invention is as follows: C: 0.05 to 0.1%, Mn: 1.5 to 2.2%, Ni: 0.00. 3 to 1.2%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, Cu: 0.1 to 0.5%, Si: 0.1 to 0.3% , P: 100 ppm or less, S: 40 ppm or less, the remainder Fe, and a slab containing other inevitable impurities is reheated to 950 to 1100 ° C., and then roughly rolled at a temperature of 1100 to 900 ° C. And finishing and rolling the bar at a temperature between Ar ₃ + 30 ° C. and Ar ₃ -30 ° C. to obtain a steel plate, and cooling the steel plate to a temperature of 700 ° C. or lower.

Slab reheating As a pre-process of rough rolling, the slab is reheated.
The reheating temperature of the slab is preferably 950 ° C. or higher, which is for dissolving the Ti and / or Nb carbonitride formed during casting. Further, in order to sufficiently dissolve Ti and / or Nb carbonitride, it is more preferable to heat to 1000 ° C. or higher. However, if reheating to an excessively high temperature, austenite may be coarsened, so the upper limit of the reheating temperature is preferably 1100 ° C.

Rough rolling Roughly rolling the reheated slab.
The rough rolling temperature is preferably equal to or higher than the temperature (Tnr) at which recrystallization of austenite stops. A cast structure such as dendrite formed during casting is destroyed by rolling, and an effect of reducing the size of austenite can be obtained. In order to obtain such an effect, the rough rolling temperature is preferably limited to 1100 to 900 ° C.
In the present invention, in order to refine the structure at the center during rough rolling, the rolling reduction per pass is 5% or more and the total cumulative rolling reduction is 40% or more for the final three passes during rough rolling. It is preferable.
In the structure recrystallized by the initial rolling during the rough rolling, crystal growth occurs at a high temperature. However, when performing the final three passes, the bar is air-cooled while waiting for rolling to slow the crystal growth rate. Accordingly, the rolling reduction ratio of the final three passes during rough rolling has the greatest influence on the grain size of the final microstructure.
Further, when the rolling reduction per pass of the rough rolling is lowered, sufficient deformation is not transmitted to the central portion, and there is a possibility that the toughness is reduced due to the coarsening of the central portion. Therefore, it is preferable to limit the rolling reduction per pass in the final three passes to 5% or more.
On the other hand, in order to refine the structure of the central portion, the total cumulative rolling reduction during rough rolling is preferably set to 40% or more.

Finish rolling The rough-rolled bar is finish-rolled at Ar ₃ (ferrite transformation start temperature) + 30 ° C. to Ar ₃ −30 ° C. to obtain a steel plate.
This is in order to obtain a more refined microstructure. When rolling is performed immediately above or below the Ar ₃ temperature, very fine ferrite is generated inside the grain boundaries and inside the crystal grains by deformation-induced transformation. Thus, the effect of reducing the crystal grain unit can be obtained.
In order to effectively cause deformation-induced transformation, the cumulative rolling reduction during finish rolling is maintained at 40% or higher, and the rolling reduction per pass excluding rolling to flatten the final shape is set to 8% or higher. It is preferable to hold.

In accordance with the conditions presented in the present invention, the grain size of the crystal grains having a high tilt boundary where the difference in crystal orientation measured by the EBSD method in the thickness direction from the surface layer portion to the thickness of the ¼ portion is 15 degrees or more during finish rolling. A microstructure that is 15 μm (micrometers) or less can be obtained.
When the finish rolling temperature is lowered to Ar ₃ -30 ° C. or less, coarse ferrite is generated before rolling, and it is elongated for a long time during rolling, and on the contrary, impact toughness is lowered, and Ar ₃ + 30 ° C. or more. When finish rolling is not effective in reducing the grain size, the finish rolling temperature is preferably in the range of Ar ₃ + 30 ° C to Ar ₃ -30 ° C.
After rough rolling, the size of crystal grains in the 1/4 t portion (here, t: steel plate thickness) of the bar before finish rolling can be 150 μm or less, preferably 100 μm or less, more preferably 80 μm or less.
After the rough rolling, the size of the crystal grains in the 1/4 t portion of the bar before the finish rolling can be controlled by the rough rolling conditions and the like.

As described above, controlling the grain size in the 1 / 4t part of the bar before rough rolling after rough rolling will refine the final microstructure due to the refinement of austenite crystal grains, improving the low temperature impact toughness. Can bring.
The reduction ratio at the time of finish rolling may be set so that the ratio of slab thickness (mm) / steel plate thickness after finish rolling (mm) is 3.5 or more, preferably 3.8 or more.
Controlling the reduction ratio as described above can increase yield / tensile strength and improve low temperature toughness due to refinement of the final microstructure as the amount of reduction increases during rough rolling and finish rolling. The toughness of the central part can be improved by reducing the particle size at the central part of the thickness.
After the finish rolling, the steel sheet has a thickness of 50 mm or more, preferably can have a thickness of 50 to 100 mm, and more preferably has a thickness of 80 to 100 mm.

Cooling After finish rolling, the steel sheet is cooled to 700 ° C. or lower.
When the cooling end temperature exceeds 700 ° C., the microstructure is not properly formed, and the yield strength may be 390 Mpa or less.
The steel sheet can be cooled at a cooling rate at the center of 1.5 ° C./s or more. When the cooling rate at the center of the steel sheet is less than 1.5 ° C./s, a fine structure is appropriately formed. There is a possibility that the yield strength becomes 390 Mpa or less.
Further, the steel sheet may be cooled at an average cooling rate of 2 to 300 ° C./s.

Below, an Example is given and this invention is demonstrated more concretely.
However, it should be noted that the examples described later are examples for explaining the present invention in more detail and are not intended to limit the scope of rights of the present invention.
This is because the scope of rights of the present invention is determined by the matters described in the claims and the matters reasonably inferred.

[Example 1]
A 400 mm steel slab having the composition shown in Table 1 below was reheated to a temperature of 1045 ° C., and then rough rolled at a temperature of 1015 ° C. to produce a bar. The cumulative rolling reduction during rough rolling was similarly applied as 50%.
The thickness of the coarsely rolled bar was 180 mm, and the size of the crystal grains in the 1/4 t portion after the rough rolling and before the finish rolling was 95 μm.
After rough rolling, finish rolling is performed at a temperature difference between the finish rolling temperature shown in Table 2 and the Ar ₃ temperature to obtain a steel sheet having the thickness shown in Table 2 below, and then 700 ° C. at a cooling rate of 4 ° C./sec. Cooled to a temperature below ℃.
For the steel plate produced as described above, the microstructure, yield strength, average grain size of 1/4 t thickness, surface parallel to the rolling direction from the surface layer portion to 1/4 part of the plate thickness in the plate thickness direction The area ratio and the Kca value (brittle crack propagation resistance coefficient) of the (100) plane forming an angle of 15 degrees or less with respect to the results are shown in Table 2.
The Kca values in Table 2 are values evaluated by performing ESSO test on steel sheets.

As shown in Table 2, in the comparative steel 1, the temperature difference of the finish rolling temperature -Ar ₃ is controlled to 50 ° C. or more during the finish rolling presented in the present invention, and sufficient reduction is not applied. The grain ratio of 4t part is 29.1 μm, and the area ratio of the (100) plane that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction from the surface layer portion to ¼ part of the plate thickness in the plate thickness direction. Is 30% or less, the impact transition temperature is −40 ° C. or higher, and the Kca value measured at −10 ° C. does not exceed 6000 required for general steel for shipbuilding.
In comparative steel 2, the C content has a value higher than the upper limit of the C content of the present invention, and despite the fact that the austenite grain size in the center is refined by cooling during rough rolling, the upper bainite ( The upper microstructure has a final fine grain size of 33.2 μm and forms an angle of 15 degrees or less with respect to a plane parallel to the rolling direction from the surface layer portion to ¼ portion of the plate thickness (100 ) The area ratio of the surface is 30% or less, and further, the upper bainite, which is easily brittle, is included as the base structure. It can be seen that it has a value.

In the comparative steel 3, the Si content has a value higher than the upper limit of the Si content of the present invention, and the grain size of the austenite at the center is refined by cooling during rough rolling. A part of upper bainite is formed, and a large amount of Si is added to form a large amount of MA structure. Therefore, the Kca value may have a value of 6000 or less at −10 ° C. I understand.
In the comparative steel 4, the Mn content has a value higher than the upper limit of the Mn content of the present invention, the fine structure of the base material is upper bainite due to the high hardenability, and the austenite at the center by cooling during rough rolling In spite of the refinement of the grain size, the final microstructure has a grain size of 32.2 μm, and an angle of 15 degrees or less with respect to the plane parallel to the rolling direction from the surface layer part to ¼ part of the plate thickness. It can be seen that the area ratio of the eggplant (100) plane is 30% or less, the impact transition temperature is −40 ° C. or higher, and the Kca value is 6,000 ° C. or lower at −10 ° C.
In the comparative steel 5, the Ni content has a value higher than the upper limit of the Ni content of the present invention, and the microstructure of the base material is granular bainite and upper bainite due to high hardenability. Although the grain size of the austenite at the center is refined by cooling, the grain size of the final microstructure is 28.7 μm, the impact transition temperature is −40 ° C. or higher, and the Kca value is −10 ° C. It can be seen that it has a value of 6000 or less.

In the comparative steel 6, the P and S contents have a value higher than the upper limit of the P and S contents of the present invention, and all other conditions satisfy the conditions presented in the present invention. It can be seen that brittleness occurs due to high P and S, and the Kca value has a value of 6000 or less at -10 ° C.
On the other hand, the invention steels 1 to 6 satisfying the component range and production range of the present invention satisfy the yield strength of 390 MPa or more and the particle size of 1/4 t part of 15 μm or less, and ferrite and pearlite structure or acicular ferrite single phase structure It can also be seen that the composite structure of acicular ferrite and granular bainite, and the composite structure of acicular ferrite and pearlite and granular bainite are fine structures.
Further, the area ratio of the (100) plane that forms an angle of 15 degrees or less with respect to the plane parallel to the rolling direction from the surface layer portion to ¼ portion of the plate thickness is 30% or more, and the impact transition temperature Is −40 ° C. or higher, and the Kca value also satisfies a value of 6000 or higher at −10 ° C.
Although the photograph which observed the thickness center part of the invention steel 6 with the optical microscope was shown in FIG. 1, the structure | tissue of thickness center part is refined | miniaturized so that FIG. 1 may also show.

表３に示したとおり、Ｃｕ／Ｎｉ重量比を適切に制御することで、鋼板の表面特性が改善されることが分かる。 [Example 2]
Except for changing the Cu / Ni weight ratio of the steel slab as shown in Table 3, a steel plate was produced with the same composition and production conditions as the inventive steel 2 of Example 1, and the surface properties of the produced steel plate were changed. The results are shown in Table 3.
In Table 3, the surface property of the steel sheet means that the presence or absence of star cracks on the surface portion due to hot shortness was measured.

As shown in Table 3, it can be seen that the surface properties of the steel sheet are improved by appropriately controlling the Cu / Ni weight ratio.

表４に示したとおり、粗圧延後のバー状態の１／４ｔにおける結晶粒の大きさが減少するほど、衝撃遷移温度が減少することが分かり、これによって、脆性亀裂伝播抵抗性が向上することが予想できる。 Example 3
A steel plate was produced with the same composition and production conditions as invented steel 1 of Example 1 except that the grain size (μm) before rough rolling was changed as shown in Table 4 after rough rolling. The impact transition temperature characteristics of the 1/4 t part of the obtained steel sheet were investigated, and the results are shown in Table 4.

As shown in Table 4, it can be seen that the impact transition temperature decreases as the crystal grain size at 1 / 4t of the bar state after rough rolling decreases, and this improves brittle crack propagation resistance. Can be expected.

  Although the present invention has been described with reference to the embodiments, those skilled in the art can make various modifications to the present invention without departing from the spirit and scope of the present invention described in the claims below. It will be understood that modifications and changes can be made.

Claims (8)

In mass% , C: 0.05 to 0.1%, Mn: 1.58 to 2.2%, Ni: 0.3 to 1.2%, Nb: 0.005 to 0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.5%, Si: 0.1-0.3%, P: 100 ppm or less, S: 40 ppm or less, remaining Fe, and other inevitable impurities Consists of
Ferrite single-phase structure, bainite single-phase structure, ferrite and bainite composite structure, ferrite and pearlite composite structure, and a microstructure consisting of one structure selected from the group consisting of ferrite, bainite and pearlite composite structure ,
The ferrite is an acicular ferrite or a polygonal ferrite, and the bainite is a granular bainite,
The steel material has a crystal grain size of 15 μm or less having a high tilt boundary where the difference in crystal orientation measured by the EBSD method in the thickness direction of the steel material from the surface layer portion to the plate thickness ¼ portion is 15 degrees or more,
The area ratio of the (100) plane forming an angle within 15 degrees with respect to the plane parallel to the rolling direction up to 1/4 part of the thickness of the steel material is 30% or more,
The steel material has a yield strength of 390 MPa or more, and a Charpy fracture surface transition temperature at a surface layer portion and a steel material thickness of 1/4 t in the thickness direction of the steel material is −40 ° C. or less,
A high-strength steel material excellent in brittle crack propagation resistance characterized by having a thickness of 50 mm or more.   The high-strength steel material excellent in brittle crack propagation resistance according to claim 1, wherein the Cu and Ni contents are set such that the Cu / Ni weight ratio is 0.6 or less.   The high-strength steel material excellent in brittle crack propagation resistance according to claim 1, wherein the fine structure of the steel material is a composite structure containing pearlite, and the fraction of pearlite is 20% or less.   The high-strength steel material excellent in brittle crack propagation resistance according to claim 1, wherein the steel material thickness is 80 to 100 mm. In mass% , C: 0.05 to 0.1%, Mn: 1.58 to 2.2%, Ni: 0.3 to 1.2%, Nb: 0.005 to 0.1%, Ti: 0.005-0.1%, Cu: 0.1-0.5%, Si: 0.1-0.3%, P: 100 ppm or less, S: 40 ppm or less, remaining Fe, and other inevitable impurities A slab made of 950 to 1100 ° C. and then roughly rolled at a temperature of 1100 to 900 ° C., and the roughly rolled bar between Ar ₃ + 30 ° C. and Ar ₃ -30 ° C. It is seen containing a step of obtaining a finish rolled to a thickness of 50mm or more of the steel sheet at a temperature, comprising the steps of cooling the steel sheet to a temperature of 700 ° C. or less, and
For the final three passes during the rough rolling, the rolling reduction per pass is 5% or more, the cumulative rolling reduction is 40% or more,
After the rough rolling, the size of the crystal grains in a 1/4 t portion (here, t: steel plate thickness) of the bar before finish rolling is 150 μm or less,
The reduction ratio during the finish rolling is set so that the ratio of slab thickness (mm) / steel plate thickness after finish rolling (mm) is 3.5 or more ,
Ferrite single phase structure, bainite single phase structure, ferrite and bainite composite structure, ferrite and pearlite composite structure, and a microstructure including one structure selected from the group consisting of ferrite, bainite and pearlite composite structure The ferrite is acicular ferrite or polygonal ferrite, and the bainite is granular bainite, from the surface layer to the thickness of 1/4 part in the thickness direction of the steel material. With respect to a plane parallel to the rolling direction up to 1/4 part of the thickness of the steel material, the grain size of the crystal grains having a high tilt boundary with a difference in crystal orientation measured by the EBSD method being 15 degrees or more is 15 μm or less. The area ratio of the (100) plane forming an angle within 15 degrees is And 0% or more, the yield strength is at least 390 MPa, as well as to manufacture a steel Charpy fracture appearance transition temperature in the surface layer and the steel material thickness 1 / 4t part is -40 ℃ less in the thickness direction of the steel A method for producing a high-strength steel material excellent in brittle crack propagation resistance. The method for producing a high-strength steel material having excellent brittle crack propagation resistance according to claim 5 , wherein the Cu and Ni contents are set such that the Cu / Ni weight ratio is 0.6 or less. . The method for producing a high-strength steel material having excellent brittle crack propagation resistance according to claim 5 , wherein the steel sheet is cooled at a cooling rate at a central portion of 1.5 ° C./s or more. The method for producing a high-strength steel material having excellent brittle crack propagation resistance according to claim 5 , wherein the steel sheet is cooled at an average cooling rate of 2 to 300 ° C./s.

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